The immune system is a relevant target for analyzing how cells interpret the myriad of signals they receive through their surface receptors to make decisions regarding their fate and/or functional responses. Alterations in these processes contribute to the development of leukemias and other cancers as well as can predispose individuals to autoimmune diseases. This project focuses on how CD4 helper T cells are generated in the thymus. It is hypothesized that protein tyrosine phosphatase receptor kappa (PTPRk), previously shown to be responsible for the defective development of CD4 T cells in LEC rats, plays a central role in CD4 lineage commitment through modulation of Lck kinase activity. To test this hypothesis, aims include to: 1. Determine whether changes in the expression levels of PTPRk alter lineage commitment;2. Show that Lck is a substrate for PTPRk;and 3. Identify the mechanisms that regulate T cell Receptor-mediated PTPRk expression during CD4 lineage commitment. PTPRk levels will be altered in T cell receptor Tg primary cells using genetic constructs, injected into recipient mice and then examined for lineage diversion. Effects on Lck activity will be tested by in vitro phosphatase assays as well as in vivo via pertinent cell lines. The expression pattern of PTPRk will be examined by quantitative RT-PCR on developmental intermediate populations. A loss of function approach will help to identify positive selection pathways affected by PTPRk. We propose a model where strong Lck signals induce PTPRK which then contributes to the sustained activation of Lck by dephosphorylating Lck at residue Y505. This positive feedback loop enables cells to commit to the CD4 lineage. Studies of T cell signaling proteins like PTPRK will expand our understanding of immune system development, especially with respect to the sequence of differentiation events that establish the functional program of helper T cells. A better understanding of T cell lineage decision and how changes in signaling between proteins may result in the formation of T cell-associated leukemias, lymphomas, and loss of tolerance leading to autoimmunity could suggest new ways to control and manipulate immune cell development for the purpose of preventing and/or treating T cell-mediated diseases.